A bibliography is a list of sources (books, journals, Web sites, periodicals, etc.) one has used for researching a topic. Bibliographies are sometimes called "References" or "Works Cited" depending on the style format you are using. A bibliography usually just includes the bibliographic information (i.e., the author, title, publisher, etc.).
An annotation is a summary and/or evaluation. Therefore, an annotated bibliography includes a summary and/or evaluation of each of the sources. Depending on your project or the assignment, your annotations may do one or more of the following.
Your annotated bibliography may include some of these, all of these, or even others. If you're doing this for a class, you should get specific guidelines from your instructor.
To learn about your topic: Writing an annotated bibliography is excellent preparation for a research project. Just collecting sources for a bibliography is useful, but when you have to write annotations for each source, you're forced to read each source more carefully. You begin to read more critically instead of just collecting information. At the professional level, annotated bibliographies allow you to see what has been done in the literature and where your own research or scholarship can fit. To help you formulate a thesis: Every good research paper is an argument. The purpose of research is to state and support a thesis. So a very important part of research is developing a thesis that is debatable, interesting, and current. Writing an annotated bibliography can help you gain a good perspective on what is being said about your topic. By reading and responding to a variety of sources on a topic, you'll start to see what the issues are, what people are arguing about, and you'll then be able to develop your own point of view.
To help other researchers: Extensive and scholarly annotated bibliographies are sometimes published. They provide a comprehensive overview of everything important that has been and is being said about that topic. You may not ever get your annotated bibliography published, but as a researcher, you might want to look for one that has been published about your topic.
From Purdue Online Writing Lab https://owl.purdue.edu/owl/general_writing/common_writing_assignments/annotated_bibliographies/index.html